The nature of the bleaching stages of rhodopsin will be investigated through the study of model systems representing the pigment chromophore and the study of rhodopsin itself. Spectra and kinetics of the bleaching stages will be determined under near physiological conditions. Of special interest is the nature of the initial steps in the bleaching process, the transition from rhodopsin to prelumirhodopsin. Special attention will also be given to the metarhodopsin I to metarhodopsin II transition to determine the nature of the transmitter released in the process of visual transduction.